The present invention relates to the inspection of components employing eddy current techniques, and more particularly, to the processing of signals from an eddy current probe array.
Any discussion of the related art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of the common general knowledge in the field.
Eddy current inspection is commonly used to detect flaws in manufactured components, such as tubes or billets. An inspection coil, typically referred to as an eddy current probe, is positioned near a piece to be inspected and driven with high frequency alternating electrical currents which, in turn, create an alternating magnetic field near the surface of the test piece. This magnetic field induces eddy currents in the conductive surface of the test piece which are sensed and measured by the eddy current probe. If a flaw or defect is present on the surface of the test piece, the flow of eddy currents will be altered, and this change will be readily detected by the eddy current probe. The amplitude and position of these current changes can then be analyzed and recorded, for example through visual inspection by a test operator or processed through an automated alarm algorithm, to determine the size and location of the defect or flaw. Eddy current array systems comprise of a plurality of inspection coils arranged in such a way as to be conducive to a particular inspection task.
Both single element and array probe eddy current inspection systems require probe balancing prior to scanning to ensure that flaw detection and sizing is accurate. Certain unavoidable variations, such as exact probe placement, manufacturing differences between coil assemblies, or environmental variables, make it impossible to predict the exact impedance readings sensed by the coil or coils in an eddy current probe for a given surface. Balancing is a process by which a reference reading for each coil in the eddy current probe is measured and recorded. This reference value is then subtracted from all subsequent measurements sensed by each coil, pulling the baseline, or null point, of each impedance reading to zero.
Complicating the issue of coil balancing in an eddy current probe is unit to unit variation among test pieces. Certain factors, such as metallurgic discrepancies or geometric variations, will affect the impedance of each test piece, and therefore result in different eddy currents for the same magnetic field. As a result, the baseline measurement will shift from test piece to test piece. This can be problematic for accurately detecting and sizing flaws.
A second complication concerning probe balancing in eddy current systems is what is typically referred to as baseline drift. In this case, metallurgic, geometric, or temperature variations, for example, along the scan path of a single test piece cause the baseline impedance reading seen by each eddy current coil in the probe to drift within the impedance plane. While these impedance variations are typically anticipated by and within the tolerance of the manufacturing process, they can limit the sensitivity of the eddy current inspection system and impede the detection of small defects.
In prior art systems, these baseline shifts—both those resulting from test piece variation and those resulting from baseline drift—were eliminated with the use of a high pass filter, which would eliminate the DC component of the measured eddy current signals, thus moving the null point of the test piece to zero regardless of the inherent impedance of the test piece, and only pass fluctuations in the measured eddy current signals, which would correspond to defects or flaws. The use of a high pass filter is an effective solution to these problems, but it also introduces a significant limitation. While brief fluctuations in the measured eddy current signal will pass through the high pass filter relatively unaltered, a significantly long defect, such as those likely to be present on a steel tube or bar, will undoubtedly be distorted. This can affect the accuracy and, in some cases, even the detection of a flaw or defect itself. Additionally, a high pass filter with a cut off frequency low enough to be of use, whether implemented digitally or in an analog circuit, would require significant resources and/or processing time.
A method proposed in U.S. Pat. No. 4,218,651 discloses a method which uses at least one eddy current probe fixed in a test head which allows the probe or probes to revolve around a test piece. This technique, and variations thereof, has become standard practice and should be well-known to those familiar with prior art. Using such a method, a defect parallel to the longitudinal axis of a test piece would be reliably measured even with a high pass filter being used to process the raw measurement data. However, such a method invariably requires a complex mechanical fixture, which will increase the cost and decrease the reliability of the test system and significantly limit the speed at which units can be tested. In addition, such a method is only useful for cylindrical test pieces.
Other related and background art can be found in U.S. Pat. Nos. 3,152,302, 4,203,069, 3,906,357, 4,673,879, 4,965,519, and 5,371,462. The contents of the aforementioned patents are incorporated by reference herein.
Accordingly it would be advantageous to provide a method of processing signals from an eddy current array which eliminated the effects of differing baseline impedances between test pieces and those of baseline drift while not distorting actual defect data. Further, it would be advantageous if this method were mechanically simple to implement and conducive to high scan rates. It would also be advantageous if this new method were applicable to bars with cross sections of geometries other than round, such as, but not limited to, oval, rectangular, and hexagonal. It would also be advantageous if this new method could be implemented without using an exceeding amount of system resources or processing time.